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CS601 - Data Communication - Lecture Handout 17

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Conversion

Digital To Analog Conversion

  • Process of changing one of the characteristics of an analog signal based on the info in a digital signal
  • When you Tx data from one computer to the other using a public telephone line
  • Original data is digital but because telephone wires carry analog signal, original data must be converted
  • Digital data must be modulated on an analog signal that has been manipulated to look like two distinct values corresponding to binary 1 to binary 0

Digital To Analog Conversion

  • Figure shows the relationship b/w digital info the digital to analog conversion hardware & resultant analog signal

Variation in Characteristics of Sine Wave

  • A sine wave is defined by 3 characteristics:
    • Amplitude
    • Frequency
    • Phase
  • By changing one aspect of a simple electrical signal back & forth,we can use it to represent digital data
  • When we vary any one of these characteristics ,we create a second version of that wave
  • If we than say that the original wave represents binary 1,the variation can represent binary 0 or vice versa
  • So by changing one aspect of a simple electrical signal back & forth,we can use it to represent digital data

Mechanisms for Modulating Digital Data to Analog Signals

  • Any of the three characteristics listed above can be altered in this way, giving us at least 3 mechanisms for modulating digital data into analog signals
    • Amplitude shift keying(ASK)
    • Frequency shift keying(FSK)
    • Phase shift keying (PSK)

Fourth Mechanism

  • In addition, there is a fourth and better mechanism that combines changes in both amplitude and phase called Quadratue Amplitude Modulation(QAM)
  • QAM is the most efficient of these options and is the mechanism used in all modern modems

Types of digital to analog modulation

Types of digital to analog modulation

Aspects of Digital to Analog Conversion

  • Before we discuss specific methods of digital to analog modulation,two basic issues must be defined:
    • Bit/Baud rate
    • Carrier signal
    • Bit Rate & Baud Rate
    • Two terms used frequently in data communication
      • Bit rate
      • Baud rate
      • Bit rate: no of bits transmitted during one second
      • Baud rate: no of signal units per second that are required to represent that bit

Bit Rate & Baud Rate

  • In discussion of computer efficiency, bit rate is more important –we want to know how long it takes to process each piece of info
  • In data transmission, however ,we are more concerned with how efficiently we can more that data from place to place, whether in pieces or blocks
  • The fewer signal units required, the most efficient the system and less bandwidth required to transmit more bits ,so we are more concerned with baud rate
  • The baud rate determines the B.W required to send the signal

Relationship b/w bit rate &band rate

  • Bit rate equals the baud rate times the no. of bits represented by each signal units
  • The baud rate equals the bit rate divided by the no. of bits represented by each signal shift
  • Bit rate is always greater than or equal to Baud rate

Analogy for Bit rate &Baud rate

  • In transportation a band is analogous to a car,a bit is analogous to a passenger
  • A car can carry one or more passengers
  • If1000 cars can go from one point to another carrying only one passenger(only driver),than 1000 passengers are transported
  • However, if each car carries four passengers, then 4000 passengers are transported
  • Note that the number of cars, not the numbers of passengers determines the traffic and therefore the need for wider highway
  • Similarly, the baud determines the required bandwidth, not the bit rate

Example 5.6

An analog signal carries 4 bits in each signal element.If 1000 signal elements are sent per second, find the Baud Rate and Bit Rate?

Solution:

  • Baud Rate= Number of Signal Elements
  • Baud Rate =1000 bauds/second
  • Bit Rate=Baud Rate * Number of bits per signal element
  • Bit Rate= 1000 * 4 = 4000 bps

Carrier Signals

  • In analog TX. The sending device produces a high frequency signal, that acts as a basis for the information signal
  • This base signal is called the Carrier Signal or Carrier Frequency
  • The receiving device is tuned to the frequency of the carrier signal that it expects from the sender
  • I=Digital info is then modulated on the carrier signal by modifying one or more of its characteristics (Amplitude, Frequency, Phase)
  • This kind of modification is called Modulation and info signal is called a Modulating Signal

Amplitude Shift Keying (ASK)

  • In ASK, the strength of carrier signal is varied to represent binary 1 or 0
  • Both frequency and phase remain constant, while the amplitude changes
  • Which voltage represents 1 and which represents 0 can be chosen by System Designer
  • A bit duration is the period of time that defines one bit
  • The peak amplitude of the signal during each bit duration is constant and its value depends on the bit (1 or 0)
  • Speed of transmission during ASK is limited by the physical characteristics of Tx. Medium

Amplitude Shift Keying (ASK)

Effect Of Noise on ASK

  • ASK is highly susceptible to noise interference
  • NOISE: Unintentional voltages introduced onto a line by various sources such as Heat or Electromagnetic Radiation from other sources
  • These unintentional voltages combine with signal to change the amplitude
  • A 1 can be changed to 0 and a 0 to a 1
  • ASK relies solely on Amplitude for recognition
  • Noise usually affects the amplitude, therefore ASK is the modulating method, that is most affected by Noise

 

  • On-Off Keying (OOK)
    • A popular ASK Technique
    • In OOK, one of the bit values is represented by no voltage
    • The advantage is the reduction in the amount of energy required to transmit Information
  • Bandwidth for ASK
    • Bandwidth of a signal is total range of frequencies occupied by that signal
    • When we decompose an ASK modulated signal, we get a spectrum of many simple frequencies
    • The most significant ones are those b/w, fc-Nbaud/2 and fc+Nbaud /2 with carrier frequency fc at the middle

Bandwidth for ASK

  • Bandwidth requirements for ASK are calculated using the formula:
  • BW=(1+d) * Nbaud
  • BW = Bandwidth
  • Nbaud= Baud Rate
  • d= factor related to condition of line (min.value = 0)

Example 5.8

Find minimum bandwidth required for an ASK signal TX at 2000 bps. TX. Mode is half duplex

Solution

  • In ASK, Baud Rate= Bit Rate
  • Therefore, Baud Rate = 2000
  • Also ASK requires a minimum bandwidth equal to its Baud Rate
  • Therefore Minimum BW = 2000 Hz

Summary

  • Digital to Analog Conversion
  • Bit Rate and Baud Rate
  • Carrier Signals
  • Amplitude Shift Keying (ASK)

Reading Sections

  • Section 5.3, “Data Communications and Networking” 4th Edition by Behrouz A. Forouzan